Insigne struck from the spot in yesterday's 4-1 win against Salernitana.
“It was important for Insigne to score this goal, it was an important moment for him," Mertens told DAZN.
“Insigne made a choice and we will miss him, because he's an important person for us. We will never forget what he did for us.
“We're a little sad, but must give him a great send-off. After 11 years here, he is Neapolitan, and we should always thank him."
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